The Milky Way’s giant black hole chews up stars and spits out planets

This artist's rendering shows planet-mass objects formed from star fragments that were shredding by the galaxy's supermassive black hole.

Mark A. Garlick/CfA

At the center of every large galaxy lies a supermassive black hole that keeps the engine turning. But when a star comes near these black holes, a massive fireworks show ensues — so massive that it can shred the star into 11,000-mass puffs of hot gas that jettison away at high speeds.

That’s pretty massive. And it’s happened in our own backyard.

At the center of the Milky Way lies Sagittarius A* (Sgr A*), a supermassive black hole. Stellar tidal disruption events likely happen every few million years. Rather than swallowing the star completely, Sgr A* rips the star apart. The end result is pretty brutal: 11,000 Jupiter-size swarms of gas.

So how will we find one?

“What will make them distinguishable from normal planets is how fast they’re moving,” Eden Girma, a sophomore at Harvard University who worked on the project, said in a press conference Friday at the 229th meeting of the American Astronomical Society.

Girma’s work focused on how close one such object might have come to our solar system. Simulations suggest that most of them are tossed out of the galaxy entirely, while a few may have stayed bound to Sgr A*. But anywhere from 1,000 to 10,000 of these objects are spewed back into the galaxy at large.

According to her simulations, though, the closest to encounter Earth might have been 700 light years away.

Finding such objects might be difficult, Girma says, because “by the time they reach us, they’ll be extremely cold.” This also might make it more difficult to characterize them. Though they will be Jupiter-mass, they won’t form by the typical mechanism of planets, where layers of gas accumulate over time. Instead, the timescale of formation could be measured in years instead of millions of years. They would be abundant in hydrogen and helium thanks to their stellar origins. Though the gas may be gravitationally bound, it may not have the density typically expected of a planet.

Even if such a planet(-ish) is found through a microlensing event, it might be hard to distinguish them from planets that formed through traditional mechanisms and were ejected out of their solar system.

For now, what we know is that there may be stragglers in the spaces between stars, remnants of a violent incident moving swiftly away from their place of origin.